Physical exercise is usually performed either as endurance exercise (aerobic exercise) characterized by continuous muscle work of large muscle groups and by a resulting feeling of shortness of breath (high pulse rate and respiration rate), sweating and general fatigue; or as strength training, characterized by intermittently high muscle tension of individual muscle groups, by performing short sequences (at most 15 muscle contractions to almost maximum muscle tension) with long rest intervals in between; of a number of different specific exercises. There are a number of technical differences between these two types of exercise, as will be explained below.
Regardless of the type of exercise involved, the internal work performed by the muscles can be divided into two different categories. In concentric work, also referred to as positive work, the muscle shortens (contracts) under an applied load. In eccentric work, also referred to as negative work, the muscle by contrast lengthens during the muscle work. For example, it is concentric work that is mainly performed when lifting a dumbbell, whereas eccentric work predominates when lowering and decelerating the weight. For a given speed of change in length, often expressed as joint angle velocity, the skeletal muscles generate a greater force in eccentric work (braking) than in concentric work (pushing). In concentric muscle work, the maximum force generated by the muscle reduces dramatically as the speed of length change (speed of contraction) increases. By contrast, the power generated by the muscle, consisting of the product of generated force and speed of movement, initially increases with increasing speed of contraction and reaches its maximum at moderately rapid concentric muscle work, when the energy requirement of the person exercising is also at its maximum. In eccentric muscle work, the power generated/the energy requirement is relatively modest.
For optimum strength-training, a high level of muscle tension is required, and preferably a limited energy requirement. Scientific studies have also shown that combined eccentric and concentric muscle work with a heavy load and at slow speeds of movement affords the greatest increase of muscle mass and strength. Consequently, a modern and high-quality strength-training apparatus provides such a load profile. SE 8900946-8 describes a method for exercising muscles by loading by means of a flywheel instead of conventional weights or dumbbells, on the one hand for the purpose of reducing the total weight of the strength-training apparatus for space flight, recreation or rehabilitation, but also for producing an optimum load profile during the eccentric phase of the muscle work.
Optimum condition-training requires, in the first place, a high energy requirement of the exercised muscles in order to achieve improved endurance of both the heart and also the peripheral muscle groups of the person doing the exercise. Higher speeds of movement are typically used compared to strength-training, thus achieving greater energy requirement and oxygen uptake. Some exercise apparatus, such as exercise bikes and rowing machines, additionally use a predominantly concentric muscle load, the eccentric muscle load being reduced by friction, thereby reducing the absolute force load.
Conventional exercise equipment cannot usually provide both strength-training and condition-training in one and the same apparatus, since, in accordance with the above description, the technical requirements differ considerably. Therefore, the construction and choice of material are generally optimized for one or other exercise regime.